Printed circuits containing one or more circuitry innerlayers are in prominent use today as demand increases for further and further weight and space conservation in electronic devices.
Successful fabricating of multi-layer printed circuit boards requires bonding together of copper and resin layers. However, direct bonding of copper and resin layers does not always provide sufficient bonding strength. It is therefore common to improve copper-resin bonding strength by providing surface roughness to the copper surface, whereby mechanical bonding between the copper and the resin layers can be enhanced. Surface roughness may be provided by mechanical cleaning, using a buffing or scrubbing machine, or by chemical cleaning.
In one method of chemical treatment, surface roughness is provided to the copper surface by depositing an oxide layer on the copper surface, such as cuprous oxide, cupric oxide, or the like. Formation of the oxide layer, which turns the pink copper surface a brown-black color, creates minute unevenness on the copper surface which provides an interlocking effect between the copper surface and resin, thus improving the bond strength.
Another method for improving the adhesion of dielectric material to a copper circuit trace uses a microetching technique. In microetching, no portion of copper, e.g., copper circuitry traces, is completely etched away. Instead, the surface is etched (or oxidized) only to a limited extent so as to leave intact the original pattern of the copper being etched. Typically, the surface of the copper is etched only to a depth of between about 20 and about 500 micro-inches, as measured from the original surface to the depths of the microetching. This is accomplished by choosing an appropriate microetching composition and limiting the extent of etching according to the parameters of the etching solution (e.g., concentration, temperature, composition, etc.).
Microetching solutions may be composed of hydrogen peroxide and an inorganic acid, such as sulfuric acid and phosphoric acid. However, these compositions can be unstable due to decomposition of the hydrogen peroxide, which results in fluctuations in the etching rate, discoloration of metals other than copper, and other problems.
Another type of microetching solution utilizes a cupric ion source, an organic acid, and a halide ion source, as described for example in U.S. Pat. No. 6,426,020 to Okada et al. and U.S. Pat. No. 5,807,493 to Maki et al., the subject matter of each of which is herein incorporated by reference in its entirety. However, these etching solutions can etch the copper surfaces more deeply than is desired.
Low metal etch depths are advantageous for at least three reasons. First, a low etch depth removes less metal from the surface thereby leaving more of the original metal cross section intact. This is particularly important for circuit traces with impedance or resistance tolerances which must be maintained since these properties are directly related to the cross sectional area of the circuit. Second, low metal etch depths allow the opportunity for reworking defective parts. Lastly, low metal etch depths reduce the rate at which metal builds up in the adhesion promoting composition. Since metal build up in the microetching composition has an effect upon the ultimate useable life of the composition, lower etch depths lead to an extended useable life for the microetching solutions in terms of the maximum square feet of metal processable per gallon of the microetching composition.
There remains a need in the art for improved microetching compositions that can provide the desired degree of microetching in a copper or copper alloy surface while overcoming some of the difficulties noted in the prior art.
To that end, the inventors of the present invention have discovered that a cupric ion-based microetching composition can be provided that reduces the degree of etching of the copper surface while providing a roughened surface with the desired surface attributes. The inventors of the present invention have discovered that the use of nitrites in the microetching composition, along with additional optional additives, provides beneficial results as compared to compositions of the prior art.